Improving photovoltaic conversion efficiency and reducing manufacturing costs have been the main drivers of the solar energy industry. A majority of commercial solar cells are made from multicrystalline or single crystal silicon wafers. Other types of solar cells based on thin film technologies such as Cu(In,Ga)Se2, CdTe, and amorphous silicon have shown great growth potential due to their lower manufacturing cost. Regardless of the technology used, there is generally a gap between the efficiency of devices produced in the lab and devices produced by mass production, mainly because of various imperfections introduced during the fabrication process. Inspecting the solar cells for defects during the fabrication processes, and finding the root causes of defects can improve the production yield and reduce manufacturing costs.
Laser beam induced current has been used to investigate solar cell defects. The method can detect various types of defects that affect the solar efficiency of a solar cell. A schematic of a laser beam induced current system 10 is shown in FIG. 1. A laser beam 12 from a laser source 14 is focused to a small spot onto the surface of a substrate 16, and a scanning device, typically an XY scanning stage or chuck 18 for moving the substrate 16 and an XY scanning mirror 20, scan the laser spot across the surface of the substrate 16 in a raster scanning scheme. The current is measured by an external circuit 22, such as a current amplifier connected to the solar cell 16. The spot scanning image is a spatial map of the efficiency of the solar cell 16 in converting light into electrical current. Dark spots in the spatial map indicate that a lower current was collected by the external measurement circuit 22, which can be caused by various types of defects in the solar cell 16. These defect types typically include light blockage at the surface of the solar cell 16, low absorption of light, low quantum efficiency, and current leakage defects (shunts).
Solar cells 16 have large capacitances that slow their response time when electric measurements are taken. In a production environment where the speed of inspection and testing are more important than the raw optical resolution, these slow-downs tend to be rather expensive. The speed of high resolution spot scanning laser beam induced current measurement is limited by the solar response time, and therefore is not generally suitable for inspecting very large substrates at a high resolution. It is also not generally suitable for integration with solar current-voltage testing, which can have an advantage in overall inspection and testing throughput and cost of ownership. Other disadvantages of spot scanning laser beam induced current include the higher cost associated with lasers, spot scanning hardware, and the complexity of the mechanical moving device.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.